I think it's a limitation on your graphics card (and drivers).Basically it's telling you your GPU doesn't have enough multitask ability. It's not uncommon in the world of graphics cards.
Something you may be able to do if it nVidia, is look for the Spanning options. There's basically 2 output modes allowed on some nvidia cards. One is dual monitor, the other is to Span monitors (one continuous desktop, including the start menu), rather than extended desktop (screens act independent of each other, second monitor has no task bar).
This may sound confusing at first, but it actually exists! Full screen applications do tend to fullscreen across both monitors

Same with Maximizing.